Resealable packages are widely used in the packaging of a variety of products, especially food. Resealable packages are convenient in that after the initial opening, consumers can close and reseal the package to better preserve the enclosed contents. Thus, a consumer avoids having to locate a storage container for the unused portion of the products in the package. As such, providing products in resealable packages appreciably enhances the marketability of those products.
Often, a resealable sealing device is produced as a separate item from the package and is attached to and made integral with the package at a later point in the manufacturing process. A common resealable sealing device is a reclosable zipper which is affixed to the package by a heat and pressure sealing process. The zipper generally includes a pressure fastenable seal in the form of a rib, or male member, located on one package wall and a mating groove, or female member, located on an opposing package wall. The interlocking male and female member of the closure arrangement are typically made of a flexible polymeric material.
Attaching a reclosable zipper to the package is fairly simple when both the zipper and the package material are made of compatible materials. By applying heat and pressure in the typical heat-sealing process, the zipper and package are made integral. A good example of this concept is a reclosable polyethylene sandwich bag utilizing a polyethylene zipper. Alternatively, in some arrangements, the packages and zipper devices are made from different materials. For example, packages made from polyester not only protect food from minor air and water losses but also minimize flavor scalping from food. But, these polyester packages often have polyethylene or polypropylene zippers. In this situation, a sealant layer between the zipper and the package is used which adequately bonds to both of the dissimilar materials. Thus, one side of the sealant layer bonds with the material of the zipper, while the other side of sealant layer bonds with the material of the package. Generally, the sealant layer is co-extruded with the closure device such that the sealant layer is connected to the closure device. The sealant layer is usually attached to a flange on the closure device. A method for co-extruding a sealant layer with a closure profile is disclosed in U.S. Pat. No. 5,242,516 issued to Custer et al. which is herein incorporated by reference.
Closure devices having an attached sealant layer are produced in long strands which are usually wound onto spools. When the closure devices are to be placed onto the packages, they are unwound from those spools and cut to the appropriate length.
However, it has been shown that the sealant material generally cannot be co-extruded with the closure profile without causing the closure device to become distorted, or curled, after the two pieces are cooled due to the differences in the coefficient of thermal expansion between the two materials comprising the sealant layer and closure device. Problems are experienced in winding the strands of closure device around the spool if the closure device has a curled portion. Furthermore, bonding the closure device to the package is much more difficult if the closure device is curled.
A need therefore exists to provide a structure at the interface between the layers of the closure devices to minimize the curling of the closure device after it has been made integral with the sealant layer when the two pieces are co-extruded.